That thin line of dirt is from idiots who don't remember to apply "wax" from the shield up toward the painted surface. If you do it the other way, the wax residue (and the dirt if you are washing your car) get under the edge, but that is hardly the end of the world. You can remove both wax residue and dirt if it were to get on the line by using a cleaner wax and if it is really bad, using a soft toothbrush to apply the cleaner wax on the edge and wiping it down.
It saves a lot of chips, it is not hard to remove but remember to remove it before it starts yellowing and crazing because then it becomes impossible to remove except with a heat gun and tweezers and will cost you more to remove than the installation. I learned my lesson the hard way on another car. Won't happen again. If you get the Lexus bra, with the Lexus guarantee, you are way ahead of the game. I discovered the error of my ways just at the end of the guarantee but the company that installed my film was out of business. I don't think Lexus will be going under before I do.